In development of a semiconductor device, particularly a semiconductor storage apparatus, memory cells are made increasingly finer to achieve a larger capacity, lower cost and the like. In a semiconductor storage apparatus mounted with a floating gate structure of a NAND flash memory device, for example, a wiring pitch between word lines to be a control gate in a gate portion is made ever finer. Such a finer structure of LSI is actively promoted for the purpose of achieving performance improvement such as faster operations of device and lower power consumption due to higher integration and curbing fabricating costs. In recent years, flash memories whose minimum processing dimension is about, for example, 20 nm are fabricated at a mass production level and still finer structures are expected to be developed, increasing technical difficulty. To realize a high-quality and high-performance semiconductor device in the face of such structures that are becoming rapidly finer, it becomes necessary to reduce variations of the threshold voltage of each memory cell or to reduce degradation of write properties or leak properties. For that purpose, for example, the height of interface between a dielectric film and an interpoly dielectric film (IPD) embedded in an device isolation region between memory cells is desirably uniform and flat in each space between memory cells.
However, a dielectric film is conventionally etched back after the dielectric film being deposited in the device isolation region between memory cells and the amount of etch-back between memory cells varies due to deteriorated accuracy of processing caused by finer structures and varied embedded film structures. Thus, interpoly dielectric films (IPD) are formed on dielectric films whose height varies, leading to non-uniform heights of interface between the dielectric film and interpoly dielectric film among spaces between memory cells. As a result, problems of threshold variations and degraded electric properties such as an increased write voltage or leak current become more evident from generation to generation.